This invention relates to the testing of devices; in particular, electrical, optical or electro-optical devices.
In the manufacture of electrical, optical and electro-optical devices it is common to subject hundreds and even thousands of devices to various time consuming and expensive tests in order to determine which ones meet predetermined specifications. The tests often include measurement of device output (e.g., voltage V or light power L) as a function of input excitation supplied, for example, in the form of input current (I). The relationships between the device inputs and outputs define functional characteristics (e.g., L-I curves or V-I curves) which themselves provide information about device performance. Take the case of a semiconductor laser for example. The L-I curve above threshold is preferably linear and has a slope equal to the differential quantum efficiency of the laser. However, certain well-known nonlinearities in the L-I curve, kinks and light jumps, are evidence that the laser will introduce noise and hence may not meet system specifications. To determine whether a laser exhibits an undesirable nonlinearity, and to quantify that nonlinearity, is no trivial task. While a few nonlinearities may be large enough to be apparent from a visual inspection of the L-I curve itself (FIG. 1), others (FIGS. 2-6) are not so self evident. In order to alleviate this problem, workers in the art have resorted to inspection of derivatives of the L-I curve becuase the derivative curves tend to emphasize deviations at the location of the nonlinearity. For example, the L-I curve of FIG. 4 contains little or no visual evidence that the laser has a kink, but the first derivative curve (dL/dI).sub.act has a large dip in the range 55-80 mA which clearly suggests the presence of one. To determine whether an undesirable kink is actually present requires that the derivative curve be compared with some reference, criterion or specification. This comparison requires that the size and location in current of the kink be quantified. Not all lasers, however, have derivative curves which exhibit kinks as clearly as FIG. 4, and even in those which do, it is still a difficult task for a test set operator to judge whether the specification is met or not.
Thus, there is a need for a technique which not only enables the accurate identification and quantification of nonlinearities in lasers and other devices, but also allows the determination to be performed rapidly so as to reduce testing time and expense.